Prior art seamless cylindrical or sleeved offset printing blanket technology is well known in the industiy and documented in several patents, for example, those assigned to Heidelberg Harris (U.S. Pat. Nos. 5,323,702; 5,429,048; 5,440,981; 5,553,541; 5,535,674 and 5,654,100) and to Reeves Brothers Inc. (U.S. Pat. No. 5,522,315) the contents of all of which patents are hereby incorporated by reference. Two examples of the prior art seamless sleeved blankets 10A, 10B are illustrated in the schematic drawings of FIGS. 1 to 3. FIGS. 2 and 3 are taken in sections parallel to the circular end of the roll. For ease of illustration, the curvature of the roll has not been shown. The FIG. 2 version 10A contains two windings of spiral wound thread 12A and is typical of blankets produced by Reeves and Day (for the Heidelberg presses). The 10A version also has a sleeve 14A, usually of nickel, the spiral wrapped threads 12A, a compressible layer 16A made of typically a rubber containing microspheres, a reinforcing layer 18A carrying another roll of spiral wrapped threads 12A, made of rubber with threads being cotton, polyester or other materials, and the printing layer 20A having a printing face 22A. Of course, the blanket including its sleeve actually curve around forming a continuous cylinder. FIG. 3 showing the version 10B, contains only one winding of spiral thread 12A and includes a thick rubber base layer 14B. This construction is typical of Sumitomo produced sleeves for use on Mitsubishi presses. This seamless cylindrical sleeve has the inner nickel sleeve 16B, a compressible layer 18B which can be joined to the base 14B by an adhesive layer 20B. A printing layer 22B is provided and has a printing face 24B. Again, the sleeve 10B actually curves around to form a seamless cylinder as shown in FIG. 1.
In the prior art, cylindrical offset sleeved printing blankets, such as discussed above, are produced by spiral winding carrier and reinforcing threads 12A/12B helically around a continuous sleeve 24A/16B. The sleeve is usually coated with an adhesion promoting primer. A first layer of polymeric coated thread is spiral wound onto the coated sleeve by passing the thread through a dip tank containing the solvated and uncured polymeric material as it is spiraled around the rotating sleeve. Dispersed in the polymeric material of this first layer are hollow microspheres that provide compressibility to the finished blanket. The amount of the coating is typically controlled as the thread exits the dip tank through a restrictive opening which must be large enough to allow the microspheres to pass through while small enough to prevent excessive coating and the resulting inability to dry and set the polymeric material before sagging can occur. The coating is relatively thick such that the solvents must be evaporated very slowly prior to curing to prevent trapped gasses from blowing unwanted voids in the finished layer. The long evaporation time tends to slow down the production rate. The polymeric material is then cured. The resulting compressible layer is very rough, uneven and overbuilt, requiring grinding to the required dimensions.
The polymeric material applied by this method tends to maintain its form around the diameter of the thread resulting in unfilled valleys between this layer and the coated sleeve. This unfilled area leads to gauge loss (thickness or diameter loss of a finished blanket sleeve—which can result in loss of printing contact) in the finished product and is sometimes compensated for by carrying out the additional steps by spreading a filling layer of solvated polymeric material onto the coated sleeve with a doctor blade set up prior to winding of the coated threads. Of course, all of the polymeric material may be applied with a doctor blade set up, as a calendered sheet or other methods known to the art and the threads omitted or spiraled around or under the applied polymeric layer.
After grinding the first inner layer to the required dimensions, a second outer layer of polymeric coated thread is wound around the sleeve in a similar fashion to the first layer; however, microspheres are not included. This layer serves as a reinforcing layer and stabilizes the overformed printing surface. Again, the polymeric material may also be applied with a doctor blade set up, as a calendered sheet or other method known to the art and the threads omitted or spiraled around or under the thus applied polymeric layer.
The overlaid printing surface may be applied as a solvated polymeric compound utilizing a doctor blade set up or as a solid by several methods known to the art such as any known extrusion or calendering process. The completed composite is cross wrapped or otherwise held in place, then cured with pressure applied to the outer layer by several methods known to the art to mold and adhere all layers together. In the final step the cured composite is again ground to the required dimensions in such a way as to provide a surface profile conducive to ink transfer.
This process results in a cylindrical offset printing blanket that is completely seamless throughout all of its layers but requires every step to be carefully performed on an individual, sleeve by sleeve basis. Efficiencies associated with mass batching of component parts are very limited, if not impossible. It has also been found that cylindrical offset printing blankets produced by this method tend to draw in the width, wrinkle or crease the paper web during use resulting in unacceptable side to side registration through successive printing units. In the prior art, to overcome this deficiency the compressible layer is profiled in a convex manner during the grinding operation to provide a spreading effect on the paper web, further requiring the individual processing of each sleeve during this step in the manufacturing process.